Cells of the Nervous System

Question 1

What are the different types of neural cells morphologically?

Answer 1

Unipolar: Has a single axonal projection
Pseudo-unipolar: A single projection that divides into two
Bipolar: Two projections
Multipolar: Multiple projections from cell body
-Pyramidal: has a pyramid shaped cell body
-Purkinje: GABA neurons found in the cerebellum
-Golgi cells: GABA neurons found in the cerebellum

Question 2

What are neurons and what common features do they have?

Answer 2

Neurons are excitable cells of the central nervous system which have a heterogeneous morphology. All have a soma (cell body, also known as perikaryon) which contains the nucleus and ribosomes as well as neurofilaments for structure and transport. Also contain an axon which is a long process that originates from the soma at the axon hillock. This can branch off into collaterals and are usually covered in myelin. Dendrites are highly branched projections of the cell body which aren’t usually covered in myelin and receive impulses from other neurons.

Question 3

List the other cells found in the CNS

Answer 3

Oligodendrocyte, Astrocyte, Microglia, Ependyma

Question 4

What are the functions of the other cells found in the CNS?

Answer 4

Oligodendrocyte - myelin-producing glial cell where each soma sends out projections forming internodes of myelin covering axons. O covers many axons but Schwann cells only myelinate one axon and is only found in the peripheral nervous system.
Astrocyte - most abundant cell in the CNS where main function is being a structural cell. Also play a key role in cell repair, synapse formation, neuronal maturation and plasticity.
Microglia - Neuronal macrophages. Perform immune functions.
Ependyma - Epithelial cells lining the fluid filled ventricles, regulating production + movement of CSF.

Question 5

How is resting membrane potential created?

Answer 5

4 major physiological ions are Na+, Ca2+, K+ and Cl- and cell membranes are impermeable to them so transport controlled by channels and pumps. Uneven distribution - more K+ in cell and more Na+ and Cl- outside cell. Ca2+ has very low conc inside cell and a higher (but still low) conc outside cell. This diff in conc of ions creates a resting potential across the membrane where the inside is more negative than the outside. RMP of between -40 to -90mV. Positive and negative charges concentrated around the membrane..

Question 6

How is an action potential created?

Answer 6

At RMP, voltage gated sodium channels and voltage gated potassium channels are both closed. However, presence of stimulus results in membrane depolarisation where the VGSCs open leading to sodium influx and more depolarisation. This influx results in VGKCs opening but at a much slower rate leading to potassium ion efflux and membrane repolarisation.

Question 7

How is ion balance restored?

Answer 7

The Na+-K+-ATPase pump restores the ion balance. In its resting configuration, Na+ enters vestibule & upon phosphorylation, ions are transported through protein. In its active configuration, as Na+ removed from cell, K+ enters the vestibule. Pump eventually returns to resting configuration and K+ is transported back into the cell

Question 8

What is saltatory conduction?

Answer 8

Myelin prevents the AP from spreading as it has high resistance and low capacitance. However, nodes of Ranvier are unmyelinated intervals along a neuron and APs jump between these nodes.

Question 9

Describe neurotransmission at a synapse

Answer 9

AP is propagated by VGSCs opening and the resulting sodium influx leads to depolarisation. This AP opens voltage-gated Ca2+ channels at the presynaptic terminal leading to calcium ion influx and vesicle exocytosis. Neurotransmitters go bind to the receptors and modulate activity in postsynaptic neurone. Then, either metabolised by enzymes in synaptic cleft or recycled by transporter proteins.

Question 10

What are the different forms of synaptic transmission?

Answer 10

Axodendritic synapse: connection between presynaptic terminal and neuronal dendrite.
Axosomatic synapse: connection between presynaptic terminal and neuronal soma
Axoaxonic synapse: connection between presynaptic terminal and neuronal axon

Question 11

What is a neuromuscular junction?

Answer 11

Specialised structure incorporating axon terminal & muscle membrane allowing unidirectional chemical communication between peripheral nerve & muscle

Question 12

How does communication at a NMJ work?

Answer 12

Paracrine as involves neurotransmitter release. Membrane depolarisation leads to influx of Ca2+ at presynaptic terminal and hence vesicles exocytose acetylcholine. NTs bind to nicotinic ACh receptors (nAChR) on skeletal muscle leading to change in end-plate potential (EPP). Miniature EPP: quantal ACh release.

Question 13

What are the disorders of the NMJ?

Answer 13

1. Botulism - Botulinum toxin (BTx): irreversibly disrupts stimulation-induced ACh release from presynaptic nerve terminal and hence contraction can’t occur.
2. Myasthenia Gravis (MG) - this is an autoimmune disorder where antibodies are directed against the ACh receptor. Cause fatigable weakness (i.e. becomes more pronounced with repetitive use)
3. Lambert-Eaton myastenic syndrome (LEMS) - Autoimmune disorder: antibodies directed against VGCC.